ph sensor with bonding agent disposed in a pattern

ABSTRACT

Embodiments described herein provide for a pH sensor that comprises a substrate and an ion sensitive field effect transistor (ISFET) die. The ISFET die includes an ion sensing part that is configured to be exposed to a medium such that it outputs a signal related to the pH level of the medium. The ISFET die is bonded to the substrate with at least one composition of bonding agent material disposed between the ISFET die and the substrate. One or more strips of the at least one composition of bonding agent material is disposed between the substrate and the ISFET die in a first pattern.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

This invention was made with Government support under contract numberN00014-10-1-0206 awarded by Office of Naval Research. The Government hascertain rights in the invention.

BACKGROUND

Researchers use sensor devices to measure pH levels in the ocean. pHlevels in the ocean are related to the amount of CO2 dissolved in theocean. By measuring the pH levels in the ocean at various depths,researchers may be able to monitor Global Warming risks and oceanhealth. Some pH sensors are capable of measuring these levels byimmersing ion sensitive field effect transistors (ISFETs) into theocean. In the oceans, there is an inverse relationship between watertemperature and pressure. Near the surface, temperatures are high andpressures are low. In deep sea, temperatures are lower but pressure ishigh. Such wide pressure variation can limit conventional pH sensoraccuracy because of the measurement errors induced by large mechanicalstresses associated in deep seas.

SUMMARY

Embodiments described herein provide for a pH sensor that comprises asubstrate and an ion sensitive field effect transistor (ISFET) die. TheISFET die includes an ion sensing part that is configured to be exposedto a medium such that it outputs a signal related to the pH level of themedium. The ISFET die is bonded to the substrate with at least onecomposition of bonding agent material disposed between the ISFET die andthe substrate. One or more strips of the at least one composition ofbonding agent material is disposed between the substrate and the ISFETdie in a first pattern.

DRAWINGS

FIG. 1 is a cross-sectional view of an embodiment of a pH sensorcomprising a bonding layer that attaches an ISFET die to a substrate.

FIG. 2 is a top view of an embodiment of the bonding layer of FIG. 1.

FIGS. 3A-3D are top views of other embodiments of the bonding layer ofFIG. 1.

FIGS. 4A and 4B are top views of yet other embodiments of the bondinglayer of FIG. 1, where the bonding layer attaches the substrate to theISFET die using a single bonding agent material.

FIGS. 5A-5E are top view of still other embodiments of the bonding layerof FIG. 1.

FIG. 6 is a top view of another embodiment of the bonding layer of FIG.1, where the bonding layer includes strips of material that arenon-orthogonal to one-another.

FIGS. 7A-7F illustrate different embodiments of a strip of bonding agentmaterial disposed in the bonding layer of FIG. 1.

FIG. 8 is a flow diagram of one embodiment of a method to form the pHsensor of FIG. 1.

FIG. 9A illustrates an embodiment of forming the substrate layer from ananisotropic single crystal form of solid.

FIG. 9B illustrates an embodiment of forming the substrate layer usingan aligned fiber composite.

FIG. 10 is a flow diagram of another embodiment of a method to form a pHsensor of FIG. 1.

FIG. 11 is an example of a mathematical model of an effect achieved bythe pH sensor of FIG. 1.

In accordance with common practice, the various described features arenot drawn to scale but are drawn to emphasize features relevant to thepresent description. Reference characters denote like elementsthroughout figures and text.

DETAILED DESCRIPTION

FIG. 1 is a cross-sectional view of an example of a pH sensor 2. The pHsensor 2 includes an ISFET die 10 having an ion sensing part 12fabricated therein for sensing a pH of a media in contact therewith. ThepH sensor 2 is configured to expose at least a portion of the ionsensing part 12 to a media (e.g., sea or ocean water) in order tomeasure a pH thereof. The ISFET die 10 has a generally planar structuredefining a first (major) surface, a second (major) surface which isreverse of the first surface, and one or more edges around the sidesbetween the first surface and the second surface. The first surface ofthe ISFET die 10 has the ion sensing part 12 fabricated therein.

The ISFET die 10 is mounted to a substrate 70 which provides mechanicalsupport to the ISFET die 10. The substrate 70 is a generally planarstructure having a third (major) surface, a fourth (major) surfacereverse of the third surface, and one or more edges around the sidesbetween the third surface and the fourth surface. The second surface ofthe ISFET die 10 is bonded to the third surface of the substrate 70. Insome examples, substrate 70 has substantially isotropic mechanicalproperties, wherein the coefficient of thermal expansion (CTE) of thesubstrate 70 in all directions parallel with a plane defined by theplanar structure is substantially the same. In such examples, thesubstrate 70 may be a ceramic formed of aluminum oxide or aluminumnitride. In other examples, the substrate 70 has anisotropic (e.g.,orthotropic) mechanical properties in directions parallel with the planedefined by the planar structure (See FIGS. 9A-9B). Substrate 70 can bemounted to a header 40. In some examples, substrate 70 is mounted to aheader with a layer 80 disposed between substrate 70 and header 40. Inan example, layer 80 may be formed of epoxy. In other examples,substrate 70 is mounted directly to header 40 without a layer of 80disposed between substrate 70 and header 40.

Substrate 70 also defines through-holes for one or more electrical pins42. One or more wires 14 provide an electrical connection between theISFET die 10 and circuitry external to die 10. Wires 14 may also bebonded to at least one electrical pin 42. In one example, embeddingwires 14 in a bonding agent material of a bonding layer 20 (discussedbelow) may provide increased protection of one or more wires 14 fromtemperature and pressure changes. Additionally, in a further example,wires 14 may be bonded to one or more electrical pins 42. A protectivevolume 43, which may comprise a partial vacuum, may be formed around thewire bond between wire 14 and at least one electrical pin 42.

In an example, pH sensor 2 also comprises cap 72 positioned around theISFET die 10. In one example, cap 72 comprises the same composition assubstrate 70. In other examples, cap 72 may comprise a differentmaterial than substrate 70. In some examples, substrate 70 and cap 72provide rigid support for the ISFET die 10 to reduce the repeatablestrains induced due to pressure and temperature changes. In someexamples, a protective layer 44 may be formed on cap 72 and part of theISFET die 10. In one example, protective layer 44 protects the pH sensorby shielding the bonding agent material of bonding layer 20 fromlong-term degradation due to exposure to salt water. In an example,protective layer 44 may be approximately chemically inert when immersedin salt water.

In the example shown in FIG. 1, pH sensor 2 comprises a frit material 22disposed in one or more areas between substrate 70 and cap 72, bondingthe substrate 70 to the cap 72. pH sensor 2 further comprises a bondinglayer 20 disposed between the ISFET die 10 and the substrate 70. Bondinglayer 20 bonds substrate 70 to the ISFET die 10. In some examples,bonding layer 20 may include one or more strips of one or morecompositions of bonding agent material. In some examples, bonding layer20 may include a homogenous composition of bonding agent material tobond the substrate 70 to the ISFET die. In other examples, bonding layer20 may include a single composition of anisotropic material. In anexample, substrate 70 may be bonded to the ISFET die 10 using thetechniques of anodic bonding, eutectic bonding or adhesive bonding.

The accuracy of conventional pH sensors may be limited by measurementerror induced by mechanical stresses associated with use in environmentssuch as deep seas, and by packaging stresses associated with making thesensor strong enough to operate over a wide pressure variation. Theseerrors may be caused by the anisotropic piezoresistance properties ofthe ISFET die 10. In particular, mechanical stresses on the ISFET die 10can alter the electrical carrier transport through the ISFET die 10.

The subject matter described herein provides a pH sensor 2 that reducespiezoresistive pH sensor errors by reducing the pressure and temperatureinduced mechanical stresses on the ISFET die 10. In particular, the pHsensors described herein maintain the piezoresistance of the ISFET diefrom the drain to the source by inducing a force on the ISFET die 10that is dependent on pressure and temperature, and counteracts at leasta portion of other pressure and temperature induced mechanical forces onthe ISFET die 10. This counteracting pressure and temperature dependentforce is induced by a difference in a coefficient of thermal expansion(CTE), or a difference in the elastic modulus, or a difference in thePoisson ratio in at least one direction between the ISFET die 10 andeither the bonding layer 20 or the substrate 70 or both; and is alsoreferred to herein as the “CTE mismatch effect.”

Turning to FIG. 11, shown in FIG. 11 is an example of a mathematicalmodel of the CTE mismatch effect over a pressure and temperature range.In the example shown in FIG. 11, State 1 and State 2 denote thepiezoresistance of the ISFET die at temperatures T₁ and T₂,respectively. As shown in FIG. 11, the CTE mismatch effect is used toreduce change in the piezoresistance from the drain to the source over apressure and temperature range. Ideally, the piezoresistance changebetween any two pressures and temperatures in the pressure andtemperature range is zero as shown in FIG. 11 (State 1≈State 2);however, in practice there will likely be some change inpiezoresistance. At State 1, the ISFET die 10 has an initialpiezoresistance coefficient matrix, π₁, at an initial temperature T₁. AtState 1, an initial stress vector, σ₁, is the stress generated on theISFET die at the pressure and temperature of state 1. Stress, σ₁, isdependent on the Elastic Modulus E₁, the Poisson Ratio PR₁, and strainε₁ of the ISFET die 10. Strain, ε₁, is dependent on coefficient ofthermal expansion CTE₁. In the example shown in FIG. 11, thepiezoresistance coefficient π of the ISFET die 10 changes with change inpressure and temperature.

As shown in FIG. 11, at T₂, the piezoresistance of the ISFET die isdependent on piezoresistance coefficient matrix π₂ and stress vector σ₂.Stress vector σ₂ is a net stress resulting from environmental stressσ_(2a) and counteracting stress σ_(2b). Environmental stress σ_(2a) isgenerated on the ISFET die 10 due to pressure and temperature variationsin the environment. The piezoresistance coefficient π₂ and theenvironmental stress σ_(2a) would result in a piezoresistance of theISFET die 10 in State 2 that is different from its piezoresistance inState 1. In order to maintain the piezoresistance of the ISFET die 10from drain to source at two different temperatures, a counteractingstress σ_(2b) can be generated on the ISFET die 10 that opposes at leasta portion of the environmental stress σ_(2a). The resulting net stressvector σ₂ is closer in value to the initial stress matrix, σ₁.

This counteracting stress σ_(2b) may be generated by tailoring eitherthe bonding layer 20 or the substrate 70 or both. In particular, thevolume change of the bonding layer 20 or the substrate 70 or both as thetemperature changes is used to induce the counteracting stress σ_(2b) onthe ISFET die 10 that maintains the piezoresistance from the drain tothe source. The counteracting stress σ_(2b) may be caused by theeffective magnitude and directional dependence of the elastic modulus(E₂), the effective magnitude and directional dependence of the Poissonratio (PR₂), or the effective magnitude and directional dependence ofthe coefficient of thermal expansion (CTE₂) on the ISFET die 10. In someexamples, the piezoresistive sensitivity of the ISFET die 10 after theCTE mismatch effect may be reduced to tenth of its initialpiezoresistive sensitivity. For example, in an ISFET die with 1-2%piezoresistance, the relative percent change in resistance due topiezoresistance, ΔR/R, from the drain to the source may be maintainedwithin 0.1-0.2% over the pressure and temperature range. In an example,the change in piezoresistance from the drain to the source may be lessthan 0.5%.

One way of achieving a CTE mismatch effect is by selecting a compositionof anisotropic material that responds to pressure and temperaturechanges by inducing forces of different magnitudes in differentdirections (See FIGS. 9-10). Such an effect is achieved by change involume due to thermal expansion/contraction of either substrate 70 orbonding layer 20 or both with respect to the ISFET die 10. In someexamples, anisotropic material is used to construct the substrate 70.Substrate 70 is oriented such that at different pressures andtemperatures the force induced by substrate 70 during thermalexpansion/contraction occurs in a direction which will counteract othermechanical forces on the ISFET die 10. In some implementations of theembodiments described with respect to FIGS. 9-10, the bonding layer 20comprises a composition of homogenous bonding agent material. In anexample, substrate 70 is orthotropic. In other examples, bonding layer20 is composed of anisotropic bonding agent material so that it respondsto pressure and temperature changes by inducing a force of differentmagnitudes in different directions to counteract other mechanical forceson the ISFET die 10. In a further example, the bonding agent materialhas orthotropic mechanical properties.

In a further embodiment of the anisotropic configuration, the CTEmismatch effect is achieved when the substrate 70 or the bonding layer20 has a CTE in one direction that is different from its CTE in a seconddirection such that at different pressures and temperatures the forceinduced by substrate 70 or the bonding layer 20 counteracts othermechanical forces on the ISFET die 10. The volume of the substrate 70 orthe bonding layer 20 changes depending on its CTE. In some examples, theelastic modulus of the substrate 70 or the bonding layer 20 is constantin all directions at varying pressures and temperatures. In someexamples, the Poisson ratio of the substrate 70 or the bonding layer 20is constant in all directions at varying pressures and temperatures.

In yet another embodiment of the anisotropic configuration, the CTEmismatch effect is achieved when the substrate 70 or the bonding layer20 has an elastic modulus in one direction that is different from itselastic modulus in a second direction such that at different pressuresand temperatures the force induced by the substrate 70 or the bondinglayer 20 counteracts other mechanical forces on the ISFET die 10. Thevolume of the substrate 70 or the bonding layer 20 changes depending onits elastic modulus. In some examples, the CTE of the substrate 70 orthe bonding layer 20 is constant in all directions at varying pressuresand temperatures. In some examples, the Poisson ratio of the substrate70 or the bonding layer 20 is constant in all directions at varyingpressures and temperatures.

In still another embodiment of the anisotropic configuration, the CTEmismatch effect is achieved when the substrate 70 or the bonding layer20 has Poisson ratio in one direction that is different from its Poissonratio in a second direction such that at different pressures andtemperatures the force induced by substrate 70 or the bonding layer 20counteracts other mechanical forces on the ISFET die 10. The volume ofthe substrate 70 or the bonding layer 20 changes depending on itsPoisson ratio. In some examples, the elastic modulus of the substrate 70or the bonding layer 20 is constant in all directions at varyingpressures and temperatures. In some examples, the CTE of the substrate70 or the bonding layer 20 is constant in all directions at varyingpressures and temperatures.

Another way of achieving a CTE mismatch effect is by selecting amaterial for the bonding layer 20 based on its CTE and disposing one ormore strips of the material between the substrate 70 and the ISFET die10 in a pattern (See FIGS. 2-8). The orientation of the one or morestrips can be selected such that the force induced by the materialduring thermal changes occurs in a direction which will counteract othermechanical forces on the ISFET die 10. In some implementations of theembodiments described with respect to FIGS. 2-8, substrate 70 hasisotropic mechanical properties. Substrate 70 may be composed of aceramic such as aluminum oxide or aluminum nitride.

In an example, the orientation of the one or more strips is selected toachieve biaxial loading of the ISFET die 10. In particular, the CTEmismatch effect can induce an orthogonal strain generated due to the CTEmismatch of material(s) of the bonding layer 20 and the ISFET die. Abeneficial biaxial force can be induced by using two differentcompositions of glass fits or bonding agents disposed between the sensordie and its mounting substrate. These compositions may be selected basedon their coefficient of thermal expansion (CTE) so that at differenttemperatures the two materials induce different thermal strains into thedie to produce biaxial loading conditions.

FIG. 2 is a top view of an embodiment illustrating an exemplary layoutof bonding layer 20 that is disposed on the second surface of the ISFETdie 10 (shown in FIG. 1). Bonding layer 20 comprises one or more stripsof bonding agent material disposed in a pattern between the secondsurface of the ISFET die 10 and the third surface of the substrate 70.In this example, the second surface (and, therefore, the ISFET 10 as awhole) has a generally rectangular shape, with a first strip 206 of afirst composition of bonding agent material disposed in a directionparallel to a long edge 207 a of the ISFET die 10. As shown in FIG. 2,multiple second strips 208 of a second composition of bonding agentmaterial are disposed orthogonally to strip 206 and parallel to a shortedge 207 b of the ISFET die 10. In this example, first strip 206 islonger than second strips 208, which are comparatively shorter anddisposed on either side of first strip 206. One or both of the first andsecond bonding agent material may be composed of a glass frit. Otherbonding agent materials may also be used for either the first strip orthe second strip. The second composition of bonding agent material usedfor the second strips 208 has a coefficient of thermal expansion (CTE)that is different from a CTE of the first composition of bonding agentmaterial used for the first strip 206. Accordingly, at differenttemperatures the two bonding agent materials will induce differentthermomechanical force onto the ISFET die 10. Since the first strip 206is oriented in a different direction (e.g., orthogonally) than thesecond strips 208, the combined forces induced by the strips 206, 208will be greater in one of the directions, which will achieve the CTEmismatch effect.

As shown in FIG. 2, an embodiment may further include a perimetersection 209 of a bonding agent material disposed along a perimeter 207of the ISFET die 10. In this example, perimeter section 209 is disposedto provide edge support and sealing. However, in another example, stripsof two different bonding agent materials with different CTE may bedisposed along the perimeter so that the thermomechanical force inducedonto the ISFET die is greater in one of the directions, and achieves theCTE mismatch effect (See FIG. 5D). Further, in the example shown in FIG.2, the bonding agent material disposed in perimeter section 209 has aCTE that is different from the bonding agent materials used for thefirst strip 206 or second strips 208. However, as shown in the followingexamples in FIGS. 3A-3D, the bonding agent material of the perimetersection may be of the same composition as one of the bonding agentmaterials of the first strip 206 or second strips 208.

FIGS. 3A-3D illustrate various examples of bonding layer 20 usingdifferent compositions of bonding agent materials along the perimeter ofthe ISFET die 10. FIG. 3A illustrates an example of bonding layer 20comprising of a pattern without a perimeter section. In FIG. 3B, thecomposition of bonding agent material used for the first strip 316 has aCTE different from the composition of bonding agent material used forthe second strips 319. The composition of bonding agent material used inperimeter section 319 does not have the same CTE as the composition ofbonding agent material used for the strip 316 or the strips 318. In FIG.3C, the composition of bonding agent material used for a perimetersection 329 along the perimeter of the ISFET die 10 has the same CTE asthe one used for the first strip 326, but not the same CTE as the oneused for the second strips 328. In FIG. 3D, the composition of bondingagent material used for a perimeter section 339 has the same CTE as thecomposition of bonding agent material used for the second strips 338,but not the same CTE as the one used for the first strip 336. In anotherexample, if a third composition of bonding agent material is used foradditional strips to achieve the CTE mismatch effect, it is to beunderstood that a perimeter section may be disposed using that thirdcomposition of bonding agent material.

FIGS. 4A-4B are embodiments of bonding layer 20 disposed between thesubstrate 70 and the ISFET die 10 (shown in FIG. 1), where the bondinglayer 20 comprises one or more strips of only one composition of bondingagent material. In an example shown in FIG. 4A, a strip 406 of acomposition of bonding agent material is disposed parallel to an edge407 a of the ISFET die 10 (shown in FIG. 1). A second bonding agent isnot used. In order to support ISFET die 10, inert material 402 isdeposited in the remaining portions of bonding layer 20. Inert material402 does not exert any CTE force on to the ISFET die 10 but may generatelimited shear stress on the die through friction. As shown in FIG. 4B anembodiment may include multiple strips 416 of a composition of bondingagent material disposed perpendicular to an edge 417 a of the ISFET die10, and inert material 412 deposited in the remaining portions ofbonding layer 20. In this example, ISFET die 10 is supported by inertmaterial 412, but may generate limited shear stress on to the ISFET die10 through friction. In the embodiments shown in FIGS. 4A and 4B, theCTE mismatch effect is achieved by orienting the strip(s) of thecomposition of bonding agent material so that the force induced by thebonding agent material counteracts with other mechanical forces on theISFET die 10. It is to be understood that in other examples the bondingagent material and the inert material may be configured in patternsother than the ones shown in FIGS. 4A and 4B to achieve the CTE mismatcheffect.

FIGS. 5A-5E are different embodiments of bonding layer 20 illustratingvarious patterns formed using compositions of bonding agent materialswith different CTE such that at different temperatures the bonding agentmaterials will induce different thermomechanical force into the ISFETdie 10 to achieve the CTE mismatch effect. For example, in FIG. 5A,first strip 506 is disposed using a composition of bonding agentmaterial that may include, but is not limited to, a glass frit. Multiplesecond strips 508 are disposed in a direction orthogonal to first strip506. The CTE of the composition of bonding agent material used todispose the multiple second strips 508 is different from the CTE of thecomposition of bonding agent material used to dispose first strip 506.At different temperatures, the combined force induced by strips 508 and506 will be greater in one direction and will achieve the CTE mismatcheffect.

In FIG. 5B, multiple first strips 516 are disposed in a directiondiagonal to edges 517 a and 517 b of the ISFET die 10. Multiple secondstrips 518 are disposed in a direction orthogonal to multiple firststrips 516 (also diagonal to edges 517 a and 517 b but in the oppositedirection). Second composition of bonding agent material used to disposemultiple second strips 518 has a different CTE from a first compositionof bonding agent material used to form multiple first strips 516, whichinduce different thermomechanical forces into the ISFET 10 at differenttemperatures in order to achieve the CTE mismatch effect. In anotherexample shown in FIG. 5C, a single strip 526 is disposed in a directiondiagonal to edges 527 a and 527 b of the ISFET die 10. Two strips 528-1and 528-2 of a second composition of bonding agent material with a CTEdifferent from the first composition of bonding agent material used todispose single strip 526 are disposed orthogonally to strip 526.

Another embodiment of bonding layer 20 is shown in FIG. 5D. In thisexample, the CTE mismatch effect may be achieved by disposing strips oftwo different bonding agent materials along the perimeter. As shown inFIG. 5D, strips 536-1 and 536-2 are disposed using a first compositionof bonding agent material. Strips 538-1 and 538-2 are disposed using asecond composition of bonding agent material. The first composition ofbonding agent material used to dispose strips 538-1 and 538-2 isdifferent from the second composition of bonding agent material used todispose strip 536-1 and 536-2. Strips 536-1 and 536-2 are orthogonal tostrips 538-1 and 538-2. At different temperatures, strips 536-1 and536-2 will induce different thermomechanical force into the die thanstrips 538-1 and 538-2 producing biaxial loading conditions to achievethe CTE mismatch effect.

FIG. 5E illustrates an embodiment of bonding layer 20. In this example,first strip 546 is disposed in a zigzag. Second strip 548 is disposed ina zigzag in the opposite direction. In an example, corners of firststrip 546 create a right angle. In an example, corners of second strip548 also create a right angle. In a further example, first strip 546 andsecond strip 548 may be orthogonal to each other. A first composition ofbonding agent material used to dispose first strip 546 has a differentCTE from a second composition of bonding agent material used to disposesecond strip 548 so that biaxial loading conditions are produced toachieve the CTE mismatch effect.

In some examples, the CTE mismatch effect may be achieved even when thestrips are disposed in a radial or an axial pattern as opposed to beingdisposed orthogonally to each other (See FIG. 6). FIG. 6 illustrates anembodiment where strips of different compositions of bonding agentmaterial may not be orthogonal to each other. Multiple first strips 606are disposed of a first bonding agent material with a CTE different froma second bonding agent material used to dispose multiple second strips608. First strips 606 and second strips 608 both induce two differentthermomechanical forces into the ISFET die 10 to produce biaxial loadingconditions in order to achieve the CTE mismatch effect. There may beadditional strips of a third bonding agent material disposed in a radialpattern, which may induce a third force onto the die and produce the CTEmismatch effect because of the difference in CTE of the differentcompositions of bonding agent materials.

FIGS. 7A-7F illustrate different embodiments of a strip of bonding agentmaterial disposed in bonding layer 20. In the present disclosure, astrip of bonding agent material refers to bonding agent material havinga narrow and elongated shape. In some examples, a strip may be a linearstrip, such as a long linear rectangular (shown in FIG. 7A) or a shortrectangle (shown in FIG. 7B). In some examples, a strip may benon-linear. For example, a strip may be an arc of uniform width (shownin FIG. 7C), a spiral (shown in FIG. 7D), or a zigzag (shown in FIG.7E). In some examples, a strip may be in waveform (square wave shown inFIG. 7F). It is to be understood that the definition of a strip is notlimited by the examples shown in FIGS. 7A-7F.

FIG. 8 is a flow diagram of one embodiment of a method 800 tomanufacture a pH sensor. As discussed herein, method 800 is describedwith respect to the examples of the pH sensor shown in FIGS. 1-7.However, method 800 may apply to other sensor examples of the pH sensoras well. In the example shown in FIG. 8, method 800 comprises formingone or more strips of first bonding agent material in a first pattern ona substrate (802). The pattern formed in block 802 may be one describedin the bonding layer embodiments of FIGS. 2-6. Block 802 may includeforming one or more strips of first bonding agent material in a patternthat is not described in the above embodiments but achieves the CTEmismatch effect. The first bonding agent material in block 802 may be aglass frit.

Method 800 further comprises forming a second material in a secondpattern on the substrate (804). Forming a second material in block 804may include depositing an inert material, or forming one or more stripsof a second bonding agent material that has a different CTE from thefirst bonding agent material used to form one or more strips in block802. In some examples, the second bonding agent material in block 804may be a glass frit. In a further example, block 804 may include formingone or more strips of the second bonding agent material orthogonally toone or more strips of first bonding agent material of block 802.

Method 800 further comprises placing the ISFET die on the substrate suchthat the first bonding agent material and the second material aredisposed between the substrate and the ISFET die (806). Method 800 alsocomprises bonding the substrate to the ISFET die by heating the bondingagent material (808). The bonding agent material in block 808 mayinclude a glass frit. In an example, heating the bonding agent materialin block 808 may include melting the glass frit using a laser basedglass frit curing technique.

In some examples, the first and second compositions of bonding agentmaterial may be two different chemical compositions of epoxy. In otherexamples, the two different compositions of bonding agent material maystart with same chemical composition of epoxy, but a filler material isadded in the epoxy to form the second composition of bonding agentmaterial such that the thermomechanical properties of the epoxy in thesecond composition are changed so that the second composition isheterogeneous, and the CTE mismatch effect is achieved. The fillermaterial used to change the composition of epoxy may be beads, sphere,fibers, or other small particles. In some examples, the filler materialmay be made of glass. In other examples, a different material may beused for the filler material.

In a different configuration, the CTE mismatch effect may be achieved byusing a substrate or a bonding layer with anisotropic mechanicalproperties. The substrate or the bonding layer has a different CTE, or adifferent elastic modulus or a different Poisson ratio in differentdirections and responds to temperature changes by inducing force ofdifferent magnitudes in different directions. In some examples, thesubstrate may be orthotropic. In some examples of this configuration,the bonding agent material disposed between the substrate and the ISFETdie may be homogenous. In some examples, the bonding layer may haveorthotropic mechanical properties. The differential force is transferredinto the ISFET die through the homogenous bonding agent producingbiaxial loading conditions to achieve the CTE mismatch effect.

In some examples, substrate 70 (shown in FIG. 1) may be constructed bygrowing a sheet of single crystal of solid material such that the solidmaterial has anisotropic mechanical properties in the single crystalform. In some examples, the substrate 70 may be orthotropic. In someexamples, the substrate 70 may be constructed from single crystalsilicon, single crystal aluminum or single crystal copper. In otherexamples, single crystal forms of other materials may be used toconstruct the substrate. FIG. 9A is a top view of substrate 70illustrating the directions and magnitudes in which the forces aregenerated. As illustrated in FIG. 9A, substrate 70 is fabricated bygrowing a sheet of anisotropic single crystal of solid material. Atdifferent temperatures the substrate generates force of two differentmagnitudes. In this example, a force 904 is generated in the directionof the y-axis and another force 908 is generated in the direction of thex-axis, which is orthogonal to the y-axis. Force 904 is of a differentmagnitude than force 908 because the CTE of the anisotropic singlecrystal of the solid material shown in FIG. 9A is different in thex-direction as opposed to the y-direction. The resulting differentialforce will achieve the CTE mismatch effect.

In other examples, substrate 70 or bonding layer 20 may be constructedof an aligned fiber composite. The fibers are intentionally aligned tocreate a composition with anisotropic mechanical properties. In someexamples, the substrate 70 may be orthotropic. In some examples, bondinglayer 20 may be formed to have orthotropic mechanical properties. Forexample, the substrate 70 or the bonding layer 20 may be formed of acarbon fiber and epoxy composite where the carbon fibers are aligned inepoxy. In some examples, the aligned fiber composite may be formed ofcarbon fibers, boron fibers, glass fibers or graphite fibers that arealigned in epoxy, resin, thermoplastic matrix or thermoset matrix. Inother examples, the aligned fiber composite may be a metal matrixcomposite that may include aluminum oxide fibers or silicon carbidefibers aligned in aluminum metal. FIG. 9B illustrates an example of thealignment of fibers to create an orthotropic composite. In theillustrated example, the preferential direction of the composite is thedirection in which the fibers are aligned. A force 918 is generated inthe preferential direction. In order to achieve the CTE mismatch effect,a force 914 orthogonal to its preferred direction is generated toproduce biaxial loading conditions.

FIG. 10 is a flow diagram of one embodiment of a method 1000 tomanufacture a pH sensor in accordance with the present description. Asdiscussed herein, method 1000 is described with respect to the examplesof the pH sensor shown in FIGS. 1 and 9A-9B. However, method 1000 mayapply to other sensor examples of the invention as well. In an example,as shown in FIG. 10, method 1000 comprises mounting a substrate on to aheader (1002). Additionally, in some examples, mounting the substrate onto the header in block 1002 may include constructing a substrate bygrowing a sheet of single crystal of a solid material such that thesolid material has anisotropic mechanical properties in its singlecrystal form. This solid material may include single crystal silicon,single crystal aluminum or single crystal copper. In other examples,mounting the substrate over the header may include constructing asubstrate of aligned fiber composite with anisotropic mechanicalproperties.

Method 1000 further comprises forming a bonding layer on to thesubstrate to bond the substrate to an ISFET die (1004). In an example,the bonding layer may include a composition of bonding agent materialthat is a glass frit. Method 1000 further comprises placing the ISFETdie on the substrate such that the bonding layer is disposed between thesubstrate and the ISFET die (1006). Method 1000 further comprisesbonding the substrate to the ISFET die by heating the bonding layer(1008).

Method 1000 further comprises configuring either the bonding layer orthe substrate or both to induce a counteracting force on the ISFET diethat opposes at least a portion of the environmental force generated onthe ISFET die due to the pressure and temperature change across thepressure and temperature range (1010). Finally, method 1000 comprisesconfiguring the counteracting force on the ISFET die to maintain thechange in piezoresistance of the ISFET die from the drain to the sourceto less than 0.5% over the pressure and temperature range (1012).

EXAMPLE EMBODIMENTS

Example 1 includes a pH sensor comprising: a substrate; an ion sensitivefield effect transistor (ISFET) die including an ion sensing part thatresponds to pH, wherein the ISFET die is bonded to the substrate;wherein the ion sensing part of the ISFET die is configured to beexposed to a medium, and wherein the ion sensing part outputs a signalrelated to a pH level of the medium; and one or more strips of at leastone composition of a bonding agent material disposed in a first patternbetween the substrate and the ISFET die.

Example 2 includes the pH sensor of Example 1, further comprising: oneor more strips of a second composition of a bonding agent materialdisposed between the substrate and the ISFET die in a second pattern,wherein the coefficient of thermal expansion (CTE) of the secondcomposition is different from the CTE of the at least one compositionsuch that at different temperatures the two materials induce forces onthe die in different directions.

Example 3 includes the pH sensor of Example 2, wherein the secondpattern further comprises one or more strips of the second compositionof bonding agent material that are disposed orthogonally to the one ormore strips disposed in the first pattern of the at least onecomposition.

Example 4 includes the pH sensor of any of Examples 1-3, furthercomprising: an inert material, wherein the inert material supports aportion of the ISFET die and does not exert a force due to CTE on thesaid portion of the ISFET die.

Example 5 includes the pH sensor of any of Examples 1-4, wherein, the atleast one composition of a bonding agent material comprises a glassfrit.

Example 6 includes the pH sensor of Example 1, wherein the ISFET die isbonded to the substrate by anodic bonding, eutectic bonding, or adhesivebonding.

Example 7 includes the pH sensor of any of Examples 1-6, furthercomprising: a third composition of a bonding agent material, wherein thethird composition of bonding agent material is disposed along theperimeter of the said ISFET die.

Example 8 includes the pH sensor of Example 7, wherein the thirdcomposition of a bonding agent material is the same as the at least onecomposition of a bonding agent material.

Example 9 includes the pH sensor of any of Examples 1-8, wherein thesubstrate comprises a base substrate and a cap formed over the basesubstrate, the pH sensor further comprising: a protective layer formedover at least a portion of an outer surface of the ISFET die and atleast a portion of the cap substrate; a cover member mechanicallycoupled to the protective layer, wherein the cover member houses theISFET die and the substrate, and wherein the cover member defines anopening proximate to the ion sensing part; a header, wherein thesubstrate is mounted to the header; a reference electrode that providesa reference voltage; and at least one electric pin coupled to the ISFETdie via a wire.

Example 10 includes the pH sensor of any of Examples 1-4, wherein the atleast one composition of a bonding agent material comprises an epoxywith filler material.

Example 11 includes a method of manufacturing a pH sensor, the methodcomprising: forming one or more strips of a first bonding agent materialon a substrate; forming a layer of second material on the substrate in asecond pattern; placing the ISFET die on the substrate such that one ormore strips of the first pattern and the layer of second material aredisposed between the substrate and the ISFET die; and bonding thesubstrate to the ISFET die by heating the first bonding agent.

Example 12 includes a method of Example 11, wherein forming a layer ofthe second material on the substrate further comprises forming one ormore strips of a second bonding agent material on a substrate in asecond pattern, wherein the coefficient of thermal expansion (CTE) ofthe second bonding agent is different from the CTE of the first bondingagent material.

Example 13 includes a method of Example 11 or Example 12, wherein thefirst bonding agent material comprises a glass frit.

Example 14 includes a method of any of Examples 11-13, wherein heatingthe first bonding agent further comprises melting the differentcompositions of frit materials using a laser based glass frit curingtechnique.

Example 15 includes a method of Example 12, further comprising: formingone or more strips of second pattern orthogonally to one or more stripsof the first pattern.

Example 16 includes a method of Example 11-15, further comprising:forming a third composition of bonding agent material, wherein thebonding agent material is formed along the perimeter of the ISFET die.

Example 17 includes a pH sensor comprising: a substrate; an ionsensitive field effect transistor (ISFET) die comprising an ion sensingpart that responds to pH, wherein the ISFET die is bonded to thesubstrate; and one or more linear strips of a first frit materialdisposed between the ISFET die and the substrate, wherein the one ormore linear stripes are all disposed in the same direction.

Example 18 includes the pH sensor of Example 17, further comprising: oneor more linear stripes of a second frit material disposed between thesubstrate and the ISFET die, wherein the one or more linear strips ofthe second frit material are disposed in a direction orthogonal to thedirection of one or more linear stripes of the said first frit material;and wherein the coefficient of thermal expansion (CTE) of the secondfrit material is different from the CTE of the first frit material suchthat at different temperatures the two materials induce force on the diein different directions to produce biaxial loading conditions.

Example 19 includes the pH sensor of Example 17 or Example 18, furthercomprising: a third frit material, wherein the frit material is formedalong the perimeter of the said ISFET die.

Example 20 includes the pH sensor of Example 19, wherein the CTE of thethird frit material is the same as the CTE of the first frit material.

What is claimed is:
 1. A pH sensor comprising: a substrate; an ionsensitive field effect transistor (ISFET) die including an ion sensingpart that responds to pH, wherein the ISFET die is bonded to thesubstrate; wherein the ion sensing part of the ISFET die is configuredto be exposed to a medium, and wherein the ion sensing part outputs asignal related to a pH level of the medium; and a plurality of strips ofat least one composition of a bonding agent material disposed in a firstpattern between the substrate and the ISFET die, wherein the pluralityof strips induce a counteracting force in a direction that counteractsat least a portion of environmental forces on the ISFET die duringthermal changes, and wherein the counteracting force is configured tomaintain the change in piezoresistance of the ISFET die from a drain toa source to less than 0.5%.
 2. The pH sensor of claim 1, furthercomprising: one or more strips of a second composition of a bondingagent material disposed between the substrate and the ISFET die in asecond pattern, wherein the coefficient of thermal expansion (CTE) ofthe second composition is different from the CTE of the firstcomposition such that at different temperatures the two materials induceforces on the die in different directions.
 3. The pH sensor of claim 2,wherein one or more strips of the second composition of bonding agentmaterial are disposed orthogonally to the plurality of strips of thefirst composition.
 4. The pH sensor of claim 1, further comprising: aninert material, wherein the inert material supports a portion of theISFET die and does not exert a force due to CTE on the said portion ofthe ISFET die.
 5. The pH sensor of claim 1, wherein the at least onecomposition of a bonding agent material comprises a glass frit.
 6. ThepH sensor of claim 1, wherein the ISFET die is bonded to the substrateby anodic bonding, eutectic bonding, or adhesive bonding.
 7. The pHsensor of claim 1, further comprising: a third composition of a bondingagent material, wherein the third composition of bonding agent materialis disposed along the perimeter of the said ISFET die.
 8. The pH sensorof claim 7, wherein the third composition of a bonding agent material isthe same as the at least one composition of a bonding agent material. 9.The pH sensor of claim 1, wherein the substrate comprises a basesubstrate and a cap formed over the base substrate, the pH sensorfurther comprising: a protective layer formed over at least a portion ofan outer surface of the ISFET die and at least a portion of the capsubstrate; a cover member mechanically coupled to the protective layer,wherein the cover member houses the ISFET die and the substrate, andwherein the cover member defines an opening proximate to the ion sensingpart; a header, wherein the substrate is mounted to the header; areference electrode that provides a reference voltage; and at least oneelectric pin coupled to the ISFET die via a wire.
 10. The pH sensor ofclaim 1, wherein the at least one composition of bonding agent materialcomprises an epoxy with filler material.
 11. A pH sensor comprising: asubstrate; an ion sensitive field effect transistor (ISFET) diecomprising an ion sensing part that responds to pH, wherein the ISFETdie is bonded to the substrate; and a plurality of linear strips of afirst frit material disposed between the ISFET die and the substrate,wherein the plurality of strips are all disposed in the same direction,wherein the plurality of strips induce a counteracting force in adirection that counteracts at least a portion of environmental forces onthe ISFET die during thermal changes, and wherein the counteractingforce is configured to maintain the change in piezoresistance of theISFET die from a drain to a source to less than 0.5%.
 12. The pH sensorof claim 11, further comprising: one or more linear stripes of a secondfrit material disposed between the substrate and the ISFET die, whereinthe one or more linear strips of the second frit material are disposedin a direction orthogonal to the direction of a plurality of linearstrips of the said first frit material; and wherein the coefficient ofthermal expansion (CTE) of the second frit material is different fromthe CTE of the first frit material such that at different temperaturesthe two materials induce force on the die in different directions toproduce biaxial loading conditions.
 13. The pH sensor of claim 11,further comprising: a third frit material, wherein the frit material isformed along the perimeter of the said ISFET die.
 14. The pH sensor ofclaim 13, wherein the CTE of the third frit material is the same as theCTE of the first frit material.